1. Field of the Invention
The present invention relates to orthodontic implements for use in correction of irregular and abnormal alignment of teeth and mismatching of upper and lower jaws, and in particular, to such implements using Ti-Ni alloy wire capable of generating a controlled correction force.
2. Description of the Prior Art
In order to correct the irregular alignment of teeth, an elastic wire element has usually been used as an element of the orthodontic implement. In detail, the wire element is bent in an appropriate or an ideal arch form and attached by brackets to the upper or lower teeth to move each one of the teeth due to the shape recovery force of the wire element.
As the elastic wire element, although a stainless steel wire has been conventionally used, a nitinol or Ti-Ni shape memory alloy wire has recently been used as disclosed in a paper entitled "Laboratory and clinical analyses of nitinol wire" by Andreasen et al, Am. J. Orthod. February 1978, pp 142-151 (reference 1) and another paper entitled "A clinical trial of alignment of teeth using a 0.019 inch thermal nitinol wire with a transition temperature range between 31.degree. C. and 45.degree. C." by Andreasen, Am. J. Orthod. November 1980, pp 528-537 Reference 2) and others.
Reference 2 discloses that nitinol wire annealed at 500.degree. C. is different from the stainless steel wire in the springback properties and is activated by mouth heat to return to its original shape. The springback property of the Nitinol is known as a pseudoelasticity. The pseudoelasticity is referred to as an unusual elastic behavior in a paper entitled "CRYSTAL STRUCTURE AND MICROSTRUCTURE OF A COLD WORKED TiNi ALLOY WITH UNUSUAL ELASTIC BEHAVIOR" by Tadaki et al, METALLURGICA, Vol. 14, 1980, Pergamon Press Ltd. pp 911-914 (Reference 3). The cold worked TiNi alloy has the pseudoelasticity and is used for the orthodontic wire element.
In correction of teeth alignment, the desired correction force is not constant for all of the teeth. In detail, providing that the desired force is one (1) for incisors of the maxillary or the upper jaw, correction forces of 1.5, 1.5, and 3.8 are desired for canines, premolars and first molars of the maxillary, respectively, and 0.7, 1.1, 1.1 and 3.2 are desired for incisors canines premolars and first molars of mandible or the lower jaw of a patient, respectively.
A typical desired correction force is about 80 grams for the incisors.
When a single orthodontic wire element is used for correction of the upper or lower teeth, it is difficult to adjust the correction force to a desired one for each one of the teeth. Accordingly, it is difficult to reduce the patient's discomfort. Also, to use different wires for different sections of teeth becomes complicated.
As another aspect of the orthodontics, it is known that a coil spring is used for correction of mismatching of the maxillary and the mandible. The coil connects the maxillary with the mandible so that the coil moves them in correct occlusal relation due to the elasticity. If the coil spring is strong, it increases the patient's discomfort, but if it is weak, correction is not sufficiently effected. Further, since the coil is subjected to a large number of repetitions of expansion and contraction, it is desirable to have high durability in order to withstand the repetition of expansion and contraction.